To be honest, I'm not expecting too much of an addition. Most likely, they'll be rather conservative about adding new Pokemon.
I'm expecting all of the cross-generational evolutions, such as Gallade and Froslass.
They might also open up some of the post-game areas during the main game, and add the Pokemon available there in Emerald to the Hoenn Dex. For example, the Desert Underpass for Ditto, or the Safari Zone's addition for Pokemon like Mareep and Houndour.
Actually, if it was like HGSS's Pokedex, then the Hoenn Pokedex wouldn't get changed at all. HGSS's updated Pokedex only included evolutions that didn't involve items - the reason being, of course, that they could place the items in Kanto (post-game) and thus you wouldn't normally be able to evolve them.
The ones it kept are all level-up w/ a certain move. Because there'd be no way to prevent you from accessing it through normal gameplay until you got to Kanto - they just learn the moves naturally. So they were added to the Johto Dex. This is why we have Mamoswine and Ambipom, but not Gliscor or Porygon-Z.
Hoenn, however, has no Pokemon like that. Every single Pokemon in Hoenn that has a cross-generational evolution or pre-evolution based on items. So, nothing would be added.